1. Field of the Invention
The present invention relates generally to column-stabilized floating structures and, more particularly, to a floating oil and gas production platform having an overall reduced response to the excitation waves imparted thereon by the seaways.
2. Description of the Prior Art
The worst expected seaway within a 100-year return period is used to design the platform and is commonly referred to as the "design seaway".
The parent application teaches a deep-drafted, floating platform, hereinafter called "the prior platform", for offshore hydrocarbon drilling and production operations in a design seaway.
The prior platform has a lower hull, an upper hull and stabilizing columns therebetween. In use in a seaway, a portion of each column is exposed to dynamic wave action. This portion is known as the dynamic wave zone of the column.
The prior platform has been designed to experience a low resultant vertical force in response to all waves with substantial energy in the design seaway. In use, the platform is moored on the production location by a conventional spread mooring system including winches, mooring lines, etc., for resisting horizontal motion in the seaway. By virtue of its low heave in the design seaway, a conventional, surface-type production wellhead tree is suspended from the prior platform The onboard wellheads are connected through production risers which extend the wellbores from the seabed. The platform's largest expected heave must be reduced so as to ensure structural integrity of the stiff production risers under the expected extreme environmental conditions in the design seaway.
In the prior platform, each column regardless of its exterior profile, has a constant waterplane area along the entire dynamic wave zone of the column that is exposed to wave action. Therefore, the prior platform will exhibit a constant waterplane area in all waves regardless of their amplitudes and in all seaways including the design seaway.
We have discovered that by taking advantage of the large variation in the amplitudes of the large number of component waves that make-up the design seaway, it is possible to further lower the platform's heave response by reducing the total active waterplane area of the columns within a portion of their dynamic wave zones.
Accordingly, it is a primary object of this invention to provide an improved platform which has a lower heave response in the design seaway as compared to the heave response of said prior platform.
It is an additional object of this invention to provide an improved platform which also has a lower heave response in seaways which are less severe than the extreme design seaway.